Page 246 - Anatomy and Physiology of Farm Animals, 8th Edition

Sense Organs / 231

(A) potentially injurious to tissue (noxious
stimuli); (4) photoreceptors, which are
VetBooks.ir the light receptors of the retina; and (5)

chemoreceptors, which respond to chem­
ical changes associated with taste, smell,
blood pH, and gas concentrations in the
blood. In normal circumstances, each
receptor is preferentially sensitive to one
type of stimulus; the unique stimulus to
which a given receptor is most sensitive is
called the adequate stimulus for that
receptor.
(B)
Receptors transduce environmental
energy into changes in membrane poten­
tial. The adequate stimulus produces a
local change in membrane potential of
the receptor, a voltage change known
as the receptor potential or generator
potential. The receptor potential for most
receptor types is depolarizing, brought
about by the opening of cation channels
permeable to Na and/or K .
+
+
The receptor potential is a graded event
that spreads passively over the local
membrane of the receptor; the amplitude
Figure 12-1. Encapsulated and nonencapsu­ of the potential change and the distance
lated receptors. (A) Naked nerve endings with along the membrane that the receptor
Golgi stain (black) in respiratory epithelium. potential travels are proportional to the
These receptors transmit information about pain. strength of the stimulus. When the stimu­
(B) Pacinian corpuscle, a type of touch receptor, lus is strong enough and the change in
has a connective tissue capsule surrounding the membrane potential reaches a critical level
terminus of the primary afferent neuron. (the threshold), an action potential begins
in the trigger zone of the sensory neuron’s
In the special and chemical senses, the peripheral process. This signal is propa­
receptor is a specialized cell separate from gated actively along the axon into the CNS
the primary afferent. These are sometimes (Fig. 12‐2) (see also Chapter 11).
specialized neurons (e.g., photoreceptors Many encapsulated receptors exhibit
in the eye) or sometimes cells that derive a physiologic characteristic called adap­
from tissues other than nervous tissue but tation. With sustained stimulation, the
which differentiate to become electrically receptors cease firing after their initial
excitable (e.g., the hair cells of the ear or burst of activity. When the stimulus is
taste cells in the tongue). withdrawn, the receptor again responds
The most functionally relevant classifi­ with a volley of action potentials. In
cation scheme for receptors is based on doing so, the adapting receptor signals
the type of stimulation to which a receptor the beginning and end of the stimulus,
best responds. Receptors in this system rather than firing throughout its duration.
may be grouped in this way: (1) mechano­ Typically, touch receptors adapt rapidly;
receptors, which respond to physical naked nerve endings – nociceptors – as a
deformation; (2) thermoreceptors, which rule do not adapt but fire continuously
respond to both heat and cold; (3) noci­ throughout application of a noxious
ceptors, which respond to stimuli that are stimulus.